1. Field of the Invention
The present invention relates to a stencil printing method using a stencil printing machine which conveys a print medium while pressing the print medium to a drum on which a stencil sheet is mounted, and transfers ink oozing from perforations of the stencil sheet onto the print medium; and a water-based ink for stencil printing that is suitable for use in this stencil printing machine.
2. Description of Related Art
The stencil printing process, unlike printing processes such as offset printing, gravure printing, and relief printing, has ease of operation and simplicity since there is no necessity for complicated operations such as washing after use and no necessity for a specialist operator. In accordance with the use of a heat sensitive plate-making method employing a thermal head as a device, in the stencil printing process, image processing is digitized, high quality printed material can be obtained simply in a short period of time, and its convenience as an information processing terminal is therefore widely acknowledged.
Printing method of a conventional stencil printing machine include an inner press printing method (refer to Japanese Patent Laid-Open Publication No. 7-132675) and an outer press printing method (refer to Japanese Patent Laid-Open Publication No. 2001-246828).
As the stencil printing ink, a water-in-oil (W/O) type emulsion ink has been generally used conventionally. W/O type emulsion inks have the function of suppressing changes in ink composition and physical properties even if the ink within the printing machine is in contact with the atmosphere when the printing machine is allowed to stand in a nonoperating state. Because of this, whereas the W/O type emulsion ink has the advantage that it has excellent storage stability, it has the problem that drying properties after printing on a print medium (substrate) such as a paper are poor.
Various improvements have been carried out in order to improve the drying properties on printed materials. For example, an ultraviolet curing type ink for stencil printing that is dried and fixed by irradiation with ultraviolet rays (Japanese Patent Laid-Open Publication No. 2002-30238) is known. Furthermore, a stencil printing method in which penetration of a water-based ink into paper is enhanced by adding a base to a printed surface immediately after printing (Japanese Patent Laid-Open Publication No. 2001-302955) is known.
However, when a drying method involving a chemical reaction is used, it is necessary to employ irradiation equipment for the curing energy, coating equipment for a reaction solution, energy therefor, etc., and it is also necessary for the ink to contain expensive starting materials.
On the other hand, in the case of a water-based ink, when the ink within a printing machine is allowed to stand for a long period of time in an open system with the printing machine in a nonoperating state, there are the problems that the water contained in the ink easily evaporates, the flowability of the ink is lost, and the composition and the physical properties of the ink change. As a result, the printing operability after the printing machine is allowed to stand for a long period of time in a nonoperating state is degraded, and printing operations cannot be carried out again without disassembling and washing the printing machine immediately after printing or immediately before resuming printing, high quality images cannot be obtained with ink that has deteriorated, and there has therefore been a desire for an improvement of these points.
It is therefore an object of the present invention to provide a stencil printing method and a water-based ink for stencil printing that can eliminate the necessity for operations such as washing even when a printing machine is allowed to stand for a long period of time in a nonoperating state, enable the same printing performance as that prior to being left standing to be obtained even after the printing machine has been allowed to stand for a long period of time, and gives printed material having excellent drying properties and excellent image properties without employing special means, equipment, energy, etc. such as applying heat, light, or a reactive material.